Harvesting the Biosphere: What We Have Taken from Nature

10. Changing Land Cover and Land Use

The most obvious, and conceptually the simplest, indicator of the human impact on the biosphere’s productivity and phytomass storage is the total area of the natural ecosystems that have been transformed by human action. To put these changes into an evolutionary context, I will begin with a brief review of phytomass storage during the past 20 millennia, since the last glacial maximum (LGM), when North America north of 50°N and much of Europe beginning at only a slightly higher latitude were covered by massive continental glaciers. Primary productivity and carbon strongly rebounded during the next 15 millennia, reaching maxima by the mid-Holocene, some 5,000–6,000 years ago at the time of the first complex civilizations. This was followed by millennia of locally severe and regionally substantial transformations whose global impact remained still relatively minor. The most common kind of conversion was to create new fields by converting forests, grasslands, and wetlands (and in some regions also deserts) to new croplands and pastures. Next in overall importance have been the claims made on forest phytomass to remove timber and firewood and, starting in the nineteenth century, wood for making paper. These human transformations of ecosystems began to accelerate during the early modern era (after 1600) and reached an unprecedented pace and extent thanks to the post-1850 combination of rapid population increases and economic growth, marked by extensive urbanization, industrialization, and the construction of transportation networks. Tracing these changes can be revealing even when most of the conclusions must be based on estimates and approximations, and a closer look is rewarded by a more nuanced understanding. At one extreme are those areas whose plant cover has been entirely lost by conversions to constructed impervious surfaces (such as pavement) in urban and industrial areas and transportation corridors: the primary production of these areas has been completely eliminated. At the other extreme are natural 10 Changing Land Cover and Land Use 158 Chapter 10 Deforestation in the tropics has been responsible for the most of the global phytomass loss during the twentieth century. This satellite image from 2010 (acquired by the Moderate Resolution Imaging Spectroradiometer on NASA’s Terra satellite) shows the extent of forest clearing in the state of Rondônia in western Brazil. The image can be downloaded at http:// earthobservatory.nasa.gov/Features/WorldOfChange/images/amazon/amazon_deforestation _2010214_lrg.jpg. Changing Land Cover and Land Use 159 grasslands that are lightly grazed by well-managed numbers of domestic animals (alpine meadows with cattle and sheep) or natural forests where harvests remove only annually renewable fruits, seeds, or nuts (such as the collection of Brazil nuts in an old-growth tropical rain forest) or are limited to a few valuable animals (such as the trapping of small mammals for their pelts in Canada’s boreal forest): the primary production of these areas remains virtually (or largely) intact. In between these two extremes is a wide continuum of human interventions. Many urban and most suburban residential areas have retained some of their site’s potential primary productivity thanks to lawns, parks, and street trees: these new anthropogenic ecosystems are highly fragmented and have a low biodiversity and low productivity when compared with their natural predecessors, but on a small scale they might approach or even surpass the performance of the ecosystems they replaced. Selective logging (including its extreme mode, which uses helicopters to remove large tree trunks from steep slopes) takes only some targeted trees, preserves much of the site’s productivity, and does not change its regenerative potential, while harvesting timber by forest clear-cutting is analogous in its destructive impact to clearing land for field crops. This most extensive of all anthropogenic land conversions is a unique hybrid of destruction and high productivity. The cultivation of annual or perennial crops is usually predicated on a near total elimination of a climax natural ecosystem, and most of the fields have lower primary productivities than the plants they replaced. But in many cases the difference in productivity is not that large, and good agronomic practices (multicropping with rotations that include high-yielding leguminous cover crops) may actually result in higher yields. The conversion of a short-grass Canadian prairie to an alfalfa field may have a minimal impact on overall primary productivity, and it will maintain such important ecosystem services as protecting soil against erosion, retaining moisture, and adding bacterially fixed nitrogen. Similarly, conversions of tropical...